Optical coupling arrangement

ABSTRACT

The invention relates to a method and an apparatus for manufacturing an optical coupling arrangement, and a specific coupling arrangement. A substrate waveguide ( 306 ) and at least one coupling element (304, 306) in the coupling arrangement are made of polymer material in the same replication process at the same time. At least one diffractive coupling element ( 304, 306 ) is then formed in the polymer material by placing the polymer material against a replication mould, which comprises a surface profile model of at least one diffractive coupling element ( 304, 306 ) that is patterned into the replication mould by means of micro lithography.

FIELD OF THE INVENTION

[0001] The invention relates to an optical coupling arrangement based ona waveguide and on the use of a diffractive coupling element forcoupling optical radiation to or from the waveguide.

BACKGROUND OF THE INVENTION

[0002] Optics plays an important part in for instance lightingtechnology, medicine, industrial measuring and monitoring applicationsand in telecommunications technology. A conventional optical systemcomprises macroscopic components placed widely apart from one another,and the size of a particularly complicated optical system may fill evena cubic metre space. In integrated optics, the aim is to combineoptoelectro-mechanical structures in order to reduce the size usingwaveguide optics, packed optics or plane integrated optics.

[0003] Integrated microtechnology often employs diffractive optics, thecomponents thereof comprising microstructures for manipulating opticalradiation as desired. A diffractive component is used for example as alens, a beam divider, an intensity distribution modifier, a mirror, anoptical safety marking, a filter, an anti-reflecting surface or apolarization modifier.

[0004] A diffractive component is used as an element in an opticalcoupling arrangement based on substrate waveguide and comprising a glasssubstrate. Diffractive elements, the optical function of which is basedon changes of the refractive index within the polymer, caninterferometrically be made on the surface of the substrate usingholographic exposure. Another alternative is to etch and metal coat thesurface of a glass substrate (or quartz substrate), in which case theoptical function of the diffractive components is based on the changesmade to the interface profile between glass and metal. Such elements canbe used to couple optical radiation between the waveguide and theenvironment. Such optical coupling arrangements based on substratewaveguide are particularly suitable as a backplane of an apparatus casein telecommunications technology, as in this way the electric datatransmission can be reduced within a circuit board and between circuitboards. Such a solution is described in greater detail for example inpublication G. Kim, R. T. Chen, Three-dimensionally interconnectedmulti-bus-line bi-directional optical back-plane, society ofPhoto-Optical Instrumentation Engineers, Opt. Eng., 38(9), pages 1560 to1566 and 1999, incorporated herein by reference.

[0005] A problem with the optical coupling arrangement based onsubstrate waveguide is that it is poorly applicable to be utilizedindustrially, as in order to manufacture a diffractive element thesubstrate must be etched and metal coated or a holographic pattern mustbe prepared. Such a work can only be carried out in a laboratory byhand, which in turn results in the fact that the coupling arrangementbecomes very expensive, is slow to manufacture and the quality is poordue to the tolerances associated with the aligning of the parts. Anextensive production to fulfil the needs of telecommunicationapplications for example is therefore not possible, as mass productionis required in industrial applications.

BRIEF DESCRIPTION OF THE INVENTION

[0006] It is an object of the invention to provide an improvedmanufacturing method, a coupling arrangement and an apparatus formanufacturing the coupling arrangement that allow simplifying themanufacture and enabling mass production without compromising thequality.

[0007] This is achieved with the method for manufacturing an opticalcoupling arrangement comprising a substrate waveguide and at least onecoupling element for coupling optical radiation between the substratewaveguide and the environment. Furthermore, the method of the inventioncomprises the steps of making the substrate waveguide and at least onecoupling element in the coupling arrangement of polymer material at thesame time in the same replication process, in which forming at least onediffractive coupling element in the polymer material by placing thepolymer material against a replication mould comprising a surfaceprofile mould of at least one diffractive coupling element that ispatterned into the replication mould by means of micro lithography.

[0008] The invention also relates to an optical coupling arrangementcomprising a waveguide structure and at least one coupling element forcoupling optical radiation between the waveguide structure and theenvironment. Furthermore, the substrate waveguide and at least onecoupling element in the coupling arrangement are made of polymermaterial and manufactured at the same time in the same replicationprocess, and at least one diffractive coupling element of the substratewaveguide is manaufactured by placing the polymer material against areplication mould comprising a surface profile model of at least onediffractive coupling element that is patterned into the replicationmould by means of micro lithography.

[0009] The invention further relates to an apparatus for manufacturingan optical coupling arrangement, the optical coupling arrangementcomprising a waveguide structure and at least one coupling element forcoupling optical radiation between the waveguide structure and theenvironment. The apparatus comprises a replication mould for polymermaterial for manufacturing the waveguide structure and at least onecoupling element associated with the waveguide structure at the samereplication time; the replication mould comprises a surface profilemodel of at least one diffractive coupling element, the surface profilemodel of the coupling element is patterned into the replication mould bymeans of micro lithography, and the apparatus is arranged to form atleast one diffractive coupling element in the polymer material of thewaveguide structure by placing the polymer material of the waveguidestructure against the replication mould and the surface profile model.

[0010] The preferred embodiments of the invention are disclosed in thedependent claims.

[0011] The invention is based on the idea that a substrate waveguide andat least one surface-patterned diffractive coupling element requiredtherein, whose function is based on the penetration of opticalradiation, are made of polymer material at the same time and in onereplication process. A diffractive coupling element couples opticalradiation to and from the waveguide as desired. The diffractive couplingelement is formed in a replication stage using a mould comprising thediffractive pattern of the polymer material.

[0012] The method and system of the invention provide severaladvantages. The manufacture of the coupling arrangement comprising awaveguide structure and coupling elements for coupling optical radiationbetween the waveguide structure and the environment is simplified, andmade more rapid and more economical. In addition, the solution makesmass production possible without having to deal with the problemsconcerning quality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In the following the invention is described in greater detail bymeans of the preferred embodiments with reference to the accompanyingdrawings, in which

[0014]FIG. 1A shows an injection moulding process before moulding,

[0015]FIG. 1B shows the injection moulding process during moulding,

[0016]FIG. 1C shows a patterned polymer layer on a base,

[0017]FIG. 1D shows a metal-clad polymer layer,

[0018]FIG. 1E shows a metal layer provided on a metal surface,

[0019]FIG. 1F shows a plate forming a diffractive pattern,

[0020]FIG. 2A shows how the diffraction efficiency of a grating iscontrolled by means of a filling factor,

[0021]FIG. 2B shows how the diffraction efficiency of the grating iscontrolled trolled by means of the groove depth,

[0022]FIG. 3 shows the function of coupling elements and a waveguide,

[0023]FIG. 4 shows how optical radiation is coupled between severalcircuit boards, and

[0024]FIG. 5 shows a multi-channel and multi-dimensional couplingarrangement.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Several replication processes of polymer material exist, such asinjection moulding and hot moulding, without being restricted thereto.FIGS. 1A and 1B show as an example the principle of an injectionmoulding method. An injection moulding arrangement according to FIG. 1Acomprises a mould including a cover 100 and a bottom 102. In addition,the injection moulding arrangement comprises a feed clamp 104 withliquid polymer 106 (or plastic). The cover 100 is provided with a model1002 for pressing or moulding microstructures, the model 1002 beingcommonly placed in a separately manufactured pattern plate, which is forinstance a printing plate or another plate comprising a diffractivepattern. In FIG. 1B the cover 100 is fastened to the bottom 102 of themould and liquid polymer mass is pressed or injected into the mould. Apart 108 resembling the mould is formed in the mould, and amicrostructure shaped according to the model 1002 is pressed onto thesurface of the part 108. Molten plastic thus shapes in accordance withthe structures of the model while being pressed against the model 1002.

[0026] In the solution shown, the part to be replicated is a substratewaveguide, on the surface of which at least one optical coupling elementcan be manufactured as a microstructure. The optical coupling elementcouples optical radiation between the substrate waveguide and theenvironment. In the replication process, a substrate waveguide in thecoupling arrangement and at least one coupling element are made ofpolymer material at the same time in such a manner that as the substratewaveguide is being manufactured one or more diffractive couplingelements are formed in the polymer material by placing the polymermaterial against a replication mould for instance by pressing, mouldingor compressing. The replication mould in turn comprises at least onediffractive coupling element model, which is patterned into the mould bymeans of micro lithography. The diffractive coupling element model isgenerally in the pattern plate, which is a part of the mould. Thus, thediffractive coupling element is specifically based on the surfacestructure forms (grooves and embosses) and on the fact that opticalradiation penetrates the diffractive coupling element, and not on therefractive index differences or reflection of the material within thecoupling element area.

[0027] Let us take a closer look how a pattern plate is manufactured inFIGS. 1C to 1F. The diffractive coupling element model is made into themould by means of micro lithography that allows making microscopicallymodulated surface profiles. A typical process is the following. A thinpolymer resist layer is spread onto a quartz or silicon wafer base oronto another corresponding base 150. The polymer resist layer issusceptible to electron radiation, ionic radiation or optical radiationmeaning that the properties thereof change during irradiation so thatthe irradiated or unirradiated area can chemically be dissolved afterirradiation.

[0028] A desired pattern can be formed onto the polymer resist layerusing a focused electron beam, an ionic beam or an optical beam so thata desired pattern is formed while the beam moves in relation to thebase. Thus, either the beam moves or the base moves. Alternatively,photon irradiation can be used through a mask including the desiredpattern either so that the mask is in close contact with the polymerresist layer or that it is optically patterned onto the polymer resistlayer. After this, the polymer resist layer is developed, or a chemicalprocess is carried out for the polymer resist layer, where the resist istotally dissolved (grating type 1, FIG. 2A) or the resist is dissolvedto the depth depending on the local amount of irradiation (grating type2, FIG. 2B).

[0029] Thus, what is known as a master structure 152 is achieved thatresembles the desired structure, but in soft polymer resist. Thisstructure is next copied onto a metallic pattern plate to enable massproduction. Then, the master structure formed from the resist is coatedwith a thin metal layer 154, and a surface conducting electricity isobtained. The coating can be carried out using evaporation orsputtering, and the thickness of the layer is normally a couple of dozennanometers at the most. In order for the pattern plate to be firm, thethin metal layer is further electrolytically provided with metal 156such as nickel. An actual pattern plate 158 is achieved by separatingthe pattern plate provided with metal from the master structure. Thethickness of the pattern plate 158 provided with nickel is typically acouple of dozens of micrometres.

[0030] The pattern plate 158 is placed into a mould that is shaped asthe desired part. When the mould is filled with plastic using a plasticupgrading technique, the pattern plate provided with nickel presses orforms otherwise wise the desired microstructures to the plasticsubstrate waveguide. The injection moulding technique is a goodalternative, if mass production is desired, but diffractive couplingelements can be manufactured also using hot moulding technique orultraviolet-hardening adhesives.

[0031] The surface-profiled grating structure in the optical couplingelement is used to couple a desired amount of optical radiation from theoptical power source to or from the waveguide, to a detector forinstance. How much optical radiation is coupled to or from the waveguidedepends on the diffraction efficiency of each grating that can becontrolled for example using the groove breadth or groove depth of thesurface profile in the coupling element.

[0032] In FIG. 2A, the changes of the filling factor affect thediffraction fraction efficiency (grating type 1). Thus, the depth of abinary grating is constant and changing the groove breadth of thegrating controls the diffraction efficiency. In general, the aim is toachieve an even optical power from a substrate waveguide 208 to thedetectors corresponding to the coupling elements (or an even power fromthe optical power sources to the waveguide). The filling factor istherefore lowest (below 0.5) for a first coupling element 200 and for alast coupling element 202 the filling factor is 0.5, referring to thefact that the breadth of the groove and the relief is equal.

[0033]FIG. 2B shows binary gratings 204 to 206, the filling factor ofwhich is constant, for example 0.5, and the diffraction efficiency iscontrolled by changing the groove depth in the grating profile (gratingtype 2). Then, the depth of the grating profile is the smallest for thefirst coupling element 204 and increases when the diffraction efficiencyis to be increased. The last coupling element 206 has the deepestgrating groove. However, the solutions shown in FIGS. 2A and 2B aremerely examples on how a desired even coupling efficiency or a desireddistribution of coupling efficiencies is achieved.

[0034] Instead of or in addition to a binary surface profile, othersurface face profile forms can also be used. For instance, a serratedpattern is possible. In fact, the grating in the coupling element can beimplemented from any pattern.

[0035]FIG. 3 shows the basic function of the substrate waveguide and thecoupling elements. An optical power source 300 radiates opticalradiation that is assembled using a lens 302. The lens 302 is notnecessary for the present solution. The optical power source 300 is forinstance a LED diode (Light Emitting Diode) or a laser that sends signalin a pulse-like manner. Optical radiation in turn refers in thisapplication to electromagnetic radiation that starts from ultravioletradiation and continues to the infrared area as a wavelength bandranging from 40 nm to 1 mm. However, the optical power source or thefunction thereof is not relevant for the present solution. The lens 302can be made of polymer material using a replication method in either thesame or a different process at the same time or at different times asthe substrate waveguide and the coupling element. The lens 302 can beplaced close to a coupling element 304 almost in contact thereto, andthe lens is separated from the coupling element 304 using nodules (notshown in FIG. 3), which are easy to manufacture for the substratewaveguide or for the lens in the replication stage. The lens can be amicrolens such as a graded refractive index GRIN lens (GRaded INdex).The lens can also be binary, in which case the lens can be integrated toform a part of the binary grating structure of the coupling element 304or the lens may be a separate component. In addition or alternativelythe lens can also be placed between the coupling element 304 and asubstrate waveguide 306.

[0036] The lens 302 employs collimation or focusing to direct opticalradiation to the coupling element 304, which transfers the opticalradiation as efficiently as desired to a substrate waveguide 306 at suchan angle that the optical radiation proceeds while being subjected tototal reflection. Optical radiation can be conveyed from the substratewaveguide using a coupling element 308, whose optical radiation radiatedinto the environment is gathered using a lens 310. The lens 310 focusesthe optical radiation to a detector 312, which is conventionally asemiconductor detector. However, the lens 310 is not essential for thepresent solution. Like the lens 302, the lens 310 can be made of polymermaterial using a replication method in either the same or a differentprocess at the same time or at different times as the substratewaveguide and the coupling element. The lens 310 can be placed close tothe coupling element 308 almost in contact thereto, and the lens isseparated from the coupling element 308 using nodules (not shown in FIG.3), which are easy to manufacture for the substrate waveguide or for thelens in the replication stage. The lens 310 can be a microlens such as agraded refractive index GRIN lens (GRaded INdex). The lens can also bebinary, in which case the lens can be integrated to form a part of thebinary grating structure of the coupling element 310 or the lens may bea separate component. The lens can also be made in a diffractive fashionor as a refracting surface profile for a uniform waveguide in themanufacturing stage in the same way as the coupling element.

[0037]FIG. 4 shows an optical backplane that allows transferring opticalsignal between several circuit boards. In this example, optical signalis sent from two circuit boards 400 and 402, and optical signal isreceived from three circuit boards 404 to 408. The optical signal iscoupled to a substrate waveguide 414 using coupling elements 410 and 412and the optical signal is connected to the detectors of the circuitboards 404 to 408 using coupling elements 416 to 420.

[0038]FIG. 5 shows a multi-channel and two-dimensional couplingarrangement. Optical power sources 500 and 502 send optical radiationtowards coupling elements 504 and 506, which couple optical radiation toa substrate waveguide 508 in such a manner that some of the opticalradiation is directed obliquely towards the back surface of thesubstrate waveguide. Optical radiation directed in such a manneradvances in the waveguide while being subjected to total reflection. Theoptical radiation advances to a coupling element 510, which couples theoptical radiation to a detector 512. Some of the optical radiation inturn falls vertically against both surfaces of the substrate waveguideand hits a detector 514. The radiation obliquely coupled to thesubstrate waveguide and the vertically penetrating radiation form twodifferent channels in different dimensions. Likewise, the opticalradiation sent by the second optical power source 502 is coupled througha coupling element 506 to the substrate waveguide 508, where the opticalradiation proceeds through the coupling element 510 to the detector 512.The optical radiation of different optical power sources proceeding tothe detector 512 is transferred in the same dimension but along adifferent channel. The channels can be separated from one another in thedetection, for instance using modulation. The optical power sources 500and 502 may be a Vertical Cavity Surface Emitting Laser VCSEL sourcewithout being restricted thereto.

[0039] Even though the invention has above been explained with referenceto the example in the accompanying drawings, it is obvious that theinvention is not restricted thereto but can be modified in various wayswithin the scope of the inventive idea disclosed in the attached claims.

1. A method for manufacturing an optical coupling arrangement comprisinga substrate waveguide (208, 306, 414, 508) and at least one couplingelement (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506, 510) forcoupling optical radiation between the substrate waveguide (208, 306,414, 508) and the environment, charaterized by comprising the steps ofmaking the substrate waveguide (208, 306, 414, 508) and at least onecoupling element (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506,510) in the coupling arrangement of polymer material at the same time inthe same replication process, in which forming at least one diffractivecoupling element (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506,510) in the polymer material by placing the polymer material against areplication mould (1000) comprising a surface profile model (1002) of atleast one diffractive coupling element (200 to 206, 304, 306, 410, 412,416 to 420, 504, 506, 510) that is patterned into the replication mouldby means of micro lithography.
 2. A method as claimed in claim 1,charaterized in that the surface profile model (1002) of the diffractivecoupling element (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506,510) is patterned into a pattern plate (158) in the replication mould bymeans of micro lithography in such a manner that a resist spread upon abase provided for the surface profile model (1002) is patterned usingradiation to conform to the diffractive coupling element (200 to 206,304, 306, 410, 412, 416 to 420, 504, 506, 510) and the pattern plate(158) including the pattern of the diffractive coupling element (200 to206, 304, 306, 410, 412, 416 to 420, 504, 506, 510) is formed on thebase using electrolysis.
 3. A method as claimed in claim 1, charaterizedin that the model (1002) of the diffractive coupling element (200 to206, 304, 306, 410, 412, 416 to 420, 504, 506, 510) is patterned into apattern plate (158) in the replication mould by means of microlithography in such a manner that the resist spread upon the baseprovided for the surface profile model (1002) is patterned usingelectron radiation, ionic radiation or optical radiation to conform tothe diffractive coupling element (200 to 206, 304, 306, 410, 412, 416 to420, 504, 506, 510) and the pattern plate (158) including the pattern ofthe diffractive coupling element (200 to 206, 304, 306, 410, 412, 416 to420, 504, 506, 510) is formed on the base using nickel electrolysis. 4.A method as claimed in claim 1, charaterized by forming several opticalchannels into the waveguide substrate (208, 306, 414, 508) in more thanone dimension and forming at least one diffractive coupling element (200to 206, 304, 306, 410, 412, 416 to 420, 504, 506, 510) for each channel.5. An optical coupling arrangement comprising a waveguide structure(208, 306, 414, 508) and at least one coupling element (200 to 206, 304,306, 410, 412, 416 to 420, 504, 506, 510) for coupling optical radiationbetween the waveguide structure (208, 306, 414, 508) and theenvironment, charaterized in that the substrate waveguide (208, 306,414, 508) and at least one coupling element (200 to 206, 304, 306, 410,412, 416 to 420, 504, 506, 510) in the coupling arrangement are made ofpolymer material and manufactured at the same time in the samereplication process, and at least one diffractive coupling element (200to 206, 304, 306, 410, 412, 416 to 420, 504, 506, 510) of the substratewaveguide (208, 306, 414, 508) is manufactured by placing the polymermaterial against a replication mould (1000) comprising a surface profilemodel (1002) of at least one diffractive coupling element (200 to 206,304, 306, 410, 412, 416 to 420, 504, 506, 510) that is patterned intothe replication mould by means of micro lithography.
 6. A couplingarrangement as claimed in claim 5, charaterized in that the replicationmould comprises a pattern plate (158) on which a surface profile model(1002) of the diffractive coupling element (200 to 206, 304, 306, 410,412, 416 to 420, 504, 506, 510) is patterned by means of microlithography in such a manner that a resist spread upon a base providedfor the surface profile model (1002) is patterned using radiation toconform to the diffractive coupling element (200 to 206, 304, 306, 410,412, 416 to 420, 504, 506, 510) and the pattern plate (158) includingthe pattern of the diffractive coupling element (200 to 206, 304, 306,410, 412, 416 to 420, 504, 506, 510) is formed on the base usingelectrolysis.
 7. A coupling arrangement as claimed in claim 5,charaterized in that the replication mould comprises a pattern plate(158) on which a surface profile mode (1002) in the diffractive couplingelement (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506, 510) ispatterned by means of micro lithography in such a manner that the resistspread upon the base provided for the surface profile model (1002) ispatterned using electron radiation, ionic radiation or optical radiationto conform to the diffractive coupling element (200 to 206, 304, 306,410, 412, 416 to 420, 504, 506, 510) and the pattern plate (158)including the pattern of the diffractive coupling element (200 to 206,304, 306, 410, 412, 416 to 420, 504, 506, 510) is formed on the baseusing nickel electrolysis.
 8. A coupling arrangement as claimed in claim5, charaterized in that the waveguide substrate (208, 306, 414, 508)comprises optical channels in more than one dimension and each channelis provided with at least one diffractive coupling element (200 to 206,304, 306, 410, 412, 416 to 420, 504, 506, 510).
 9. An apparatus formanufacturing an optical coupling arrangement, the coupling arrangementcomprising a waveguide structure (208, 306, 414, 508) and at least onecoupling element (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506,510) for coupling optical radiation between the waveguide structure(208, 306, 414, 508) and the environment, charaterized in that theapparatus comprises a replication mould (1000) for polymer material formanufacturing the waveguide structure (208, 306, 414, 508) and at leastone coupling element (200 to 206, 304, 306, 410, 412, 416 to 420, 504,506, 510) associated with the waveguide structure (208, 306, 414, 508)at the same replication time, the replication mould (1000) comprises asurface profile model (1002) of at least one diffractive couplingelement (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506, 510), thesurface profile model (1002) of the coupling element (200 to 206, 304,306, 410, 412, 416 to 420, 504, 506, 510) is patterned into thereplication mould (1000) by means of micro lithography, and theapparatus is arranged to form at least one diffractive coupling element(200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506, 510) in thepolymer material of the waveguide structure (208, 306, 414, 508) byplacing the polymer material (1002) of the waveguide structure (208,306, 414, 508) against the replication mould (1000) and the surfaceprofile model (1002).
 10. An apparatus as claimed in claim 9,charaterized in that the replication mould (1000) comprises a patternplate (158) patterned as the surface profile model (1002) into thereplication mould by means of micro lithography, the pattern plate beingmanufactured in such a manner that a resist spread upon a base providedfor the surface profile model (1002) is patterned using radiation toconform to the diffractive coupling element (200 to 206, 304, 306, 410,412, 416 to 420, 504, 506, 510), and the pattern plate (158) includingthe pattern of the diffractive coupling element (200 to 206, 304, 306,410, 412, 416 to 420, 504, 506, 510) is formed of the base usingelectrolysis.
 11. An apparatus as claimed in claim 9, charaterized inthat the replication mould (1000) comprises a pattern plate (158)patterned as the surface profile model (1002) into the replication mouldby means of micro lithography, the pattern plate being manufactured insuch a manner that a resist spread upon a base provided for the surfaceprofile model (1002) is patterned using electron radiation, ionicradiation or optical radiation to conform to the diffractive couplingelement (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506, 510), andthe pattern plate (158) including the pattern of the diffractivecoupling element (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506,510) is formed of the base using nickel electrolysis.
 12. An apparatusas claimed in claim 9, characterized in that the replication mould(1000) comprises a pattern plate (158) as the surface profile model(1002), on which the surface profile model (1002) in the diffractivecoupling element (200 to 206, 304, 306, 410, 412, 416 to 420, 504, 506,510) is patterned, and the apparatus is arranged to press the patternplate (158) of the replication mould against the polymer material inorder to form the diffractive coupling element (200 to 206, 304, 306,410, 412, 416 to 420, 504, 506, 510).
 13. An apparatus as claimed inclaim 9, characterized in that the apparatus is arranged to form severaloptical channels to the waveguide substrate (208, 306, 414, 508) in morethan one dimension and the apparatus is arranged to form at least onediffractive coupling element (200 to 206, 304, 306, 410, 412, 416 to420, 504, 506, 510) for each optical channel.